1. Field of the Invention
The invention is generally related to valveless metering pumps for delivering precise volumes of fluid and is specifically related to a microfluid pump for precisely dispensing reagents in assay tests.
2. Description of the Prior Art
It is known to use assay testing to determine the presence of infectious diseases such as hepatitis, syphilis and the HIV virus in the presence of blood serum. In a typical procedure, a precise volume of a biological sample is disposed in a test receptacle and a reagent is added to the sample to perform an immunoassay using an automated analyzer. Typically, the reagent is carried on latex microparticles and is delivered in precise volume to the test sample. The reagent volume for each sample can be in the range of 50 to 100 microliters and must be dispensed within a plus or minus 0.5 microliter accuracy and precision and with less than one percent coefficient of variance.
It has become common practice that each pump may deliver a specific reagent to each of one or more test sample locations and, in the prior art, a valve mechanism is used to control the flow of the reagent from first one station and then to the other.
Because of the high precision requirements of pump systems for delivering reagents, the drop size, the condition of the meniscus at the end of the outlet ports and the pressure variation due to valve movement must all be taken into consideration to assure accurate test samples. For example, the minuscule pumping action inherent in shifting a valve from one position to another is of critical significance when dealing with the volumes commonly associated with assay type testing. This, coupled with the requirement that the components of the pump which come into contact with the reagent must be of an inert material such as tetrafluro plastics and/or ceramics or the like has led to very expensive and complex designs. Unfortunately, the more complex the design the greater the likelihood for error in manufacturing and assembly, further increasing the cost by requiring tight tolerances to minimize the effect of tolerance stacking. In addition, more complex systems with the associated number of moving parts contribute to field failure and maintenance cost.
More recently, valveless, positive displacement metering pumps have been successfully employed in applications where safe and accurate handling of fluids is required. The valveless pumping function is accomplished by the simultaneous rotation and reciprocation of a piston in a work chamber. The pump head containing the work chamber and piston is mounted such that is may be swiveled with respect to the rotating drive. The degree of angle controls the stroke and length and in turn, the flow rate. This type of pump has been found to be useful in performing accurate transfers of both gaseous and liquid fluids.
An example of a valveless positive displacement pump is disclosed in U.S. Pat. No. 4,008,003. The pump includes a cylinder divided into a pair of working chambers, each of the chambers communicating with an inlet and an outlet port. The pump disclosed in the U.S. Pat. No. 4,008,003 patent does not lend itself to accurate calibration for metering and dispensing fluids in the precise volumes called for in assay type tests. The piston stroke is not easily adjusted and the angular displacement of the ports cannot be readily calibrated. Another example of a valveless metering pump using a tiltable housing to control the piston stroke disclosed in my co-pending application entitled Pump with Multi-Port Discharge, Ser. No. 07/463,260, filed Jan. 10, 1990, now U.S. Pat. No. 5,015,157 with the co-inventors R. W. Jaekel and D. Pinkerton.